This invention relates to color cathode ray tubes and more particularly to a tube screen structure providing improved contrast of the color image display.
Cathode ray tubes of the type employed in color television applications for presenting multi-color display imagery, usually utilize patterned screens that are comprised of repetitive groups of related phosphor materials. Such groupings are normally disposed as bars, stripes or dots depending upon the type of color tube structure under consideration. For example, in the well known shadow mask tube construction, the screen pattern is conventionally composed of a vast multitude of repetitive tri-color emissive areas formed of selected cathodoluminescent phosphors, which, upon predetermined excitation, produce additive primary hues to provide the desired color imagery. Associated with the screen and spaced therefrom is a foraminous structure or patterned mask member having a vast multitude of discretely formed apertures therein. Each of the apertures in the mask member is related to a specific tri-component grouping of related phosphor areas of the screened pattern in a spaced manner therefrom to enable the selected electron beams traversing the apertures to impinge the proper screen areas therebeneath.
Several approaches have been proposed to increase the contrast ratio of the color screen display. On such proposal to improve contrast by absorbing ambient light is the use of a neutral density filter member in the form of a tinted cover plate which is superposed on the viewing panel of the tube. Since neutral density filters are not appreciably selective in the visible band of the color spectrum, the intended absorptive efficiency can not be fully realized in eliminating the reflective ambient light falling within the spectrum bandpass of the display emission. Another proposal for increasing contrast ratio of the color image display is the utilization of a tinted faceplate or viewing panel per se. Tinting of the faceplate attenuates the light transmission of that member, thereby reducing the evidenced brightness of the phosphor emissions, in addition to absorptive shortcomings similar to those of the aforementioned neutral density filter.
Another approach to improving contrast of the color screen image, particularly in a dot-type patterned screen, has been the development of a screen structure wherein the dot-defining interstitial spacing between the respective color-emitting dots of the screen pattern is formed of an opaque light absorbing material. In essence, each phosphor dot is enclosed or defined by a substantially dark encompassment which collectively comprise a foraminous pattern in the form of a windowed webbing having an array of substantially opaque connected interstices. While this black surround feature reduces the reflected ambient light in the non-fluorescing areas of the screen, it does not reduce the ambient light reflected from the panel areas associated with the phosphor dots, which areas evidence a high degree of reflectivity.
A further proposal for enhancing contrast of the screen display has been the use of optical filter elements disposed relative to the respective color-emitting phosphors comprising the screen pattern. On such optical filter proposal utilizes circular filter elements having large oversize diameters dimensioned so that their outer peripheral portions overlap in a non-uniform manner to produce an irregularly shaped or indented filter area surrounded by a non-uniform interstitial webbing. These variations in the dot surround-webbing effect a variable absorbency of the ambient light thereby detracting from the complete achievement of the intended contrast enhancement.